Chapter 6 Structures Part 650Engineering Field Handbook

NEBRASKA AMENDMENT (Formerly EFM)

(Engineering Field Manual Notice NB-26, May 1981) NE 6-34g

CATHODIC PROTECTIONMAINTENANCE TESTING

Cathodic protection systems need to bechecked at least once every two to fouryears to make sure they are functioning.Testing the system is relativelystraightforward, but special equipment isnecessary to perform the test. Eachcathodic protection system has a testbox(es) installed to facilitate checking thesystem.

Anodes are a very low voltage-current flowsystem with a voltage and current flow lessthan that of a 2-cell flashlight. As the anodecorrodes to supply the small current, it isbeing 'sacrificed' to keep the pipe fromcorroding and, eventually, is used up to thepoint where the anode, like a battery,ceases to function. Problems most likely tooccur in a system will be a broken wire, badwire connections, or a broken weld. Aschematic of a typical system looks like this:

Chapter 6 Structures Part 650Engineering Field Handbook

NEBRASKA AMENDMENT (Formerly EFM)

NE 6-34h (Engineering Field Manual Notice NB-26, May 1981)

(a) The four tests normally usedfor checking the system are:

(1) Test the pipe-to-soil voltagepotential by comparing to a coppersulfate half-cell.

The copper sulfate half-cell is a specializedhalf of a battery cell which, when placed in

contact with the ground and connectedthrough a volt meter to a metal in contactwith the ground, will produce a comparativevoltage for the metal to soil. Thecomparative voltage for a corrugated metal(iron) pipe that is protected should be 0.85volt or higher. Less than 0.80 means thepipe probably is corroding.

(2) Testing continuity of the pipe.

The pipe should act like one large wire fromriser to the last fully buried joint so thatcurrent added from the anodes will followthrough and protect the total length of thepipe. Since a weld under a fill on a smallpipe is almost impossible to correct, the

pipe is thoroughly checked before backfill iscompleted. This is done with an ohmmeterwith a small internal battery to supply thelow voltage. The same procedure can beused to check for breaks in a pipe circuit asthe pipe ages. See schematic on followingpage.

Chapter 6 Structures Part 650Engineering Field Handbook

NEBRASKA AMENDMENT (Formerly EFM)

(Engineering Field Manual Notice NB-26, May 1981) NE 6-34i

Test "A" will show if the pipe is making acontinuous circuit.Test "B" will show if either of the wiresbetween the test box and the pipe is brokenor has a bad connection to the pipe.

Ohm readings of less than 3 ohms indicatecontinuity.

Chapter 6 Structures Part 650Engineering Field Handbook

NEBRASKA AMENDMENT (Formerly EFM)

NE 6-34j (Engineering Field Manual Notice NB-26, May 1981)

(3) The voltage output of the anode.This test is a comparative test using thecopper sulfate half-cell as in Test 1. Thenormal anode potential voltage will be in therange of 1.4 to 1.6 volts. A 0 to 0.3 voltagereading means the wire or a connection isbroken between the test box and the anode.

Sometimes this can be corrected by diggingto the base of the post, back toward theanode, and checking the wire for breaks,since the break often is at the base of thepost due to the post having been knockedover. A schematic of the test is:

(4) The measuring of the current flowfrom the anode(s) to the pipe.

This could vary from 5 ma (0.005 amp) to300+ ma (0.3 amp), depending on pipe area,soils resistivity, and soil moisture. A currentflow larger than the design current usuallymeans the pipe is protected to a higher level

than needed and a waste of the anode,which will result in a reduced anode life. Ifthe current is more than 1.3 times thedesign needs, a resistance should be addedto the circuit to extend the life of the anodes.This is done with a length of special high-resistance wire between the connections inthe test box. Schematic follows:

Chapter 6 Structures Part 650Engineering Field Handbook

NEBRASKA AMENDMENT (Formerly EFM)

(Engineering Field Manual Notice NB-26, May 1981) NE 6-34k

The following basic equipment is needed formaintenance testing:

� 250 ft to 300 ft of rubber-coated #18wire on a reel

� 1 volt ohm-milliamp (VOM) meter

with - DC volt ranges of 0 to 2.5 or 3 volts- milliamp ranges of 0 to 500 with 2 lower maximum deflection ranges- ohm ranges with expanded scale in the 1 to 5+/- ohm range

� 1 copper-copper sulfate half-cellwith porous plugThe half-cells are available fromany supplier of corrosionengineering testing equipment.

� 2 or 3 extra 4- to 8-ft leads of #18wire with small alligator clips